It's lunchtime at Wimbledon Park primary school in south London. Cheerful pop music wafts through the brightly coloured dining hall and children tuck into their school dinners with gusto. On the menu today is white fish topped with cheddar cheese, quorn and vegetable noodles, new potatoes and a medley of summer vegetables.

But it hasn't always been like this. Three years ago, only 30 children out of 400 were eating school dinners. Without a kitchen to prepare fresh food, chilled and frozen produce was heated in the school's oven. Vegetables were a watery slush and processed meat was served regularly. Headteacher Dee Russell says: "You can't imagine how disgusting it was. The smell was awful."

The picture is now so perfect that you almost expect Jamie Oliver to pop out of the kitchen and praise the "wicked" transformation. Undoubtedly, his 2005 television programme, Jamie's School Dinners, which exposed the state of many school meals in Britain, helped create the climate for change. However, the real progress here was made without television cameras, celebrities or money, by a grassroots organisation, Merton Parents for Better Food in Schools.

Local parent Jackie Schneider set up the group after being galvanised into action by Oliver's programme. "We decided it was time to stop moaning and start acting," she says. "Suddenly, there was a spotlight on the issue. I thought it would be a flash in the pan, so decided to capitalise on it."

After a long, and sometimes tortuous, dispute with Merton council and contract caterers, the group has succeeded in getting healthy, fresh food into all 43 primary schools in the borough.

The war is not won yet. Today marks the launch of phase two of the campaign - changing the menu in the borough's secondary schools. With four of its eight such schools locked into 25-year private finance intiative (PFI) contracts, that is a more daunting challenge. But the group, which began as a loose coalition of concerned individuals, is now a lobby group of national importance. "I'm confident we'll win," Schneider says. "We have the precedent of the primary school campaign and the support of the local authority now. We can't afford to lose."

Schneider, 44, talks with the confidence of a seasoned campaigner, but three years ago she was simply a teacher at a primary school, and a mother worried about the food her three children were being served at school. Along with two like-minded parents, Chris Larkman and Erica Cooke, she contacted every headteacher in the borough and asked them to spread the word about a meeting. She expected a smattering of disgruntled parents, but more than 150 parents, governors and teachers turned up. "People were so frustrated and sick of being fobbed off and ignored," she says. "There was a legacy of inertia that had to be challenged. We had to prove we were different and we were not going to back down."

The group's aims were modest: to draw attention to the poor quality of food in schools and to make the council remedy the situation. "If they had just given us a few more tomatoes and a bit of lettuce, we probably would have shut up and gone away," she admits.

Instead, the council went on the defensive. A front page article in the Guardian, citing the Merton campaign, provoked a strident response from the then leader of the council, Andrew Judge, who claimed that the authority worked closely with contractors to offer a "healthy, nutritious range of meals". Schneider says: "Everybody was incensed. We couldn't believe how badly informed he was. People had been suppressed for so long that the lid just came off."

Despite having little experience of campaigning, Schneider and her fellow protesters began a flurry of activity. More than 100 children and parents stood outside Merton civic centre, holding up pictures of the school meals alongside the question: "Are you eating what we are eating?" A petition attracted 3,000 signatures, stalls were set up in the high street, councillors were lobbied, and parents kept informed.

The campaigners also began collecting evidence. Dinner ladies, such as Rachael Stayton, took surreptitious photographs of the food in Merton schools, and they were posted on the Merton Parents website. "The majority of us were 100% behind what Jackie was doing," Stayton says. "The quality of the food and the [small] portion sizes were just appalling. Any mother would have said, 'No way is my child eating that.'" Schneider contacted local and national press, and within days the photographs were on the front page of the Daily Mirror.

Some other evidence-gathering methods were less conventional. When the local authority claimed the fish fingers served in schools were 40% fish, Schneider was informed by a dinner lady "insider" that the true figure was 20%. Using a step ladder, she climbed up and rifled in the bins for the packaging, and took the incriminating evidence to the council. "The caterers started taking packaging away with them at the end of the day," she says. "It was unbelievable."

As a teacher, and indirect employee of the authority, did Schneider ever fear for her job? "I only ever acted as a parent," she says. "I was very careful not to do anything I could be sacked for." She describes being made to feel "very uncomfortable" by certain members of the authority, one of whom accused her of threatening school meals services. But she never lost her resolve. "I wasn't at all frightened," she says.

After the initial onslaught, and only a month after their first meeting, Merton Parents was invited to meet with the council to try to resolve the problem. Yet progress was limited and tokenistic, with a proposed new menu woefully lacking in healthy and nutritious food, Schneider says. "All they wanted to do was more of the same, but that wasn't working."

The group was invited on to a school meals development group - if they agreed to sign confidentiality agreements. They refused. "It was absolutely essential that we stayed on the outside and retained our independence," she says.

Inadequate response

Dave Hill, whose appointment as director of children's services at Merton council in December 2005 resulted in a more constructive relationship with the parents group, admits that the council's response was inadequate. "People had just accepted the situation," he says. "It took a kick up the behind from the parents for the council to sit up and take notice." And it was clear from his first meeting with Schneider and Merton Parents that the group would not be placated: "It was obvious they wouldn't go away. I either had to get on board and listen to them or have them harassing me for my entire time as director of children's services."

Healthier meals were introduced in September 2006, resulting in a 10% increase in school meal take-up in the borough, but the end of the primary school catering contract in July 2007 provided a real opportunity to get the group's aims and objectives incorporated into a new, more stringent contract. Despite concerns in the local authority that no one would bid, eight companies competed.

The council now employs a full-time nutritionist and monitors the menu in all schools under its care. Would this have happened had it not been for the intransigent parents? "Not in a million years, if I'm perfectly honest," Hill admits. "The parents made all the difference."

But can Merton Parents achieve its goal of healthier meals, an end to long dinner time queues, and a table space and chair for every child in secondary schools? The campaign has the backing of the local authority, but 25-year PFI contracts in the four secondary schools in the borough limits its bargaining power.

Ironically, the authority may instead have to rely on some of the methods used by Merton Parents if those goals are to be met. Hill says: "Merton now has a reputation for healthy eating, and we have to exploit that. If the catering company come on board, the publicity will be great; if they don't, it will be bad. Our leverage is in the margins."

The Merton campaign has come a long way since its humble beginnings. In October last year, it held a national conference for parents, with big name delegates such as schools secretary Ed Balls, environmentalist Zac Goldsmith, and famous "foodie" Prue Leith.

The group is regularly asked to support other parent groups. Helen Clark Bell, a Merton Parents campaigner with a background in contract law, describes how she recently sat up late into the night helping a group of parents in Lewisham, south London, with their new contract. "We want to offer our experience so they don't have to reinvent the wheel," she says. "But we can only offer advice; we don't have the solutions. For a grassroots campaign to work, it has to be run by the people on the ground."

Schneider has seen her role change from a local leader to unofficial spokeswoman for parents throughout the country. In March 2006, she met Gordon Brown ("for a nanosecond!") after winning an award for consumer action from the Sheila McKechnie Foundation for campaigners, and she has advised the government's School Food Trust and its Healthy Schools programme for England.

Exit strategy

Now Merton Parents is forming an exit strategy, hopeful that, after the high school campaign, the checks and monitoring the group has helped establish will mean that the school meals system will continue to run smoothly.

So will this mark the end of Schneider's campaigning? That seems unlikely: she is already talking about attacking something "not so easily popular, not photogenic or cuddly" next time. She says: "I try to be a good teacher. But even at my best I impact on the lives of 30 children. As a campaigner, I can have a much bigger impact and I get a real buzz from that."

Asked what she thinks about being described variously as formidable, a maverick and a revolutionary (the last an accolade from Leith), Schneider laughs. "I'm not at all formidable," she says. "And I think it is strange to be described as a revolutionary for expecting such a simple basic right as a decent school lunch for every child." She stops, thinks for a second, and then says: "Maverick I can live with, because unthinking conformity is dangerous. There is nothing wrong with challenging orthodoxies."